Spinal cord ischemia resulting in paralysis and paresis may occur after trauma, during surgical correction of spine deformities, during major vascular surgery, following pain management procedures, and may also occur as a result of vascular disorders. Hypotension, hypoxia, vasospasm, inflammation, edema, and hemorrhage all propagate spinal ischemia after injury. Prevention of secondary injury from ischemia may limit disability. Preventing secondary injury to the spinal cord, as with the brain, requires continued hemodynamic support and often, early surgical intervention. Prevention of secondary spinal injury is focused upon the preservation or restoration of spinal cord blood flow and oxygen delivery. No existing method, however, is available for measuring or monitoring the impact of interventions upon these critical parameters.
In the cases of cerebral ischemia and traumatic brain injury, improvement in outcome has come from an improved ability to monitor, in real time, the success of interventions such as angioplasty, thrombolysis, intracranial pressure, and cerebral oxygenation. Efforts focused upon ameliorating spinal cord injury have thus far met with dismal results.
Currently, the only methods employed for assessment of spinal cord ischemia are based on electrophysiology, including both somatosensory (SSEP) and motor (MEP) evoked potentials. These technologies monitor the integrity of posterior spinal somatosensory and anterior/lateral spinal motor tracts, respectively; when combined, they can identify injury, offer insight into the impact of particular interventions, and thereby provide an opportunity to limit or reverse injury. Such neuro-electrophysiological monitoring, however, may be not infrequently influenced by anesthetic management, patient temperature, limb ischemia and technological malfunctions (lead displacement, disconnection, etc.). Furthermore, accurate and timely interpretation of these data requires skilled neurologists with expertise in neuro-electrophysiological monitoring. “False negatives”, wherein patients have awakened with serious deficits in spite of “normal” evoked potentials, and “false positives”, wherein patients have awakened without deficits in spite of loss or degradation of signal, have both been reported by SSEP and MEP monitoring. Finally, abnormalities in neuro-electrophysiological measurements may be delayed relative to the inciting event, which diminishes chances for rescue of threatened tissues. Despite these limitations, MEP and SSEP remain the “gold standard” for functional monitoring of the spinal cord during aortic, spine, and spinal cord surgery.
Tools available to measure spinal cord blood flow are extremely limited. The ability to measure spinal cord blood flow with laser Doppler has been demonstrated in both animal and human studies. These devices, however, measure flow in a very limited tissue volume, in close proximity to the probe tip. LDF sampling volumes are estimated at 0.3-0.5 mm3. Additionally, positioning of the rigid probe is troublesome, the probes are prone to fracture, and they cannot be left in place for an extended period of time. Noninvasive methods for spinal cord blood flow measurement have been superficially investigated; they include single photon emission computed tomography and MRI based arterial spin labeling. MRI and CT may become excellent tools for the measurement of spinal cord blood flow and can even be expected to have superior spatial sensitivity. Intraoperative monitoring with both MRI and CT, however, is simply not feasible and these tools certainly would not allow for continuous monitoring. Indeed, no current approach can be reasonably applied in a field surgical hospital.
The ability to measure spinal cord blood flow and oxygenation would: 1) aid in the ability to expeditiously diagnose and monitor the progress of spinal cord ischemia; 2) offer an enhanced opportunity to prevent secondary injury; 3) allow the assessment of the efficacy of interventions aimed at ameliorating ischemia; 4) assist in early surgical stabilization of spinal trauma; 5) allow for the continuous assessment of spinal cord blood flow and oxygenation for several days after surgery; and 6) assist in the laboratory and clinical assessment of the efficacy of novel therapeutic approaches to ameliorate ischemia. Finally, the ability to combine MEP/SSEP with blood flow/oxygenation monitoring would provide key additional insight into the mechanism of injury. Accordingly, there are needs in the art for devices capable of measuring spinal cord blood flow and or oxygenation and also for related methods.